This invention relates to linear actuators for positioning the heads of disk files.
Transducing heads for reading information from and writing information on the information storage disks of a disk file must be accurately positioned over selected concentric information bearing tracks on the disk surface. Various types of actuator have been employed to position the heads of which the most common is the linear voice coil actuator.
This comprises a coil which is situated in a annular air gap in a magnet assembly such that energization of the coil causes it to move linearly along the air gap. The coil is connected to and supported by a carriage which also supports the heads. The carriage runs on guide ways or surfaces which constrain it to move in a linear path radially of the disks.
A recent example of such an actuator can be found in U.S. Pat. No. 4,661,729. In that patent the guide ways are horizontally spaced parallel rods between which the carriage runs on three pairs of outrigger bearings. The conventional material for such guide rods is steel and an example of another such actuator in which steel is specified as the guide rod material is described in published European patent application EP 0223610 A2.
The use of magnetic steels to form guide rods has been recognized to have potential disadvantages in the case of voice coil actuators since their magnetization by the voice coil magnet assembly can affect actuator performance by causing the carriage bearings, if also magnetic, to "cog" or run unevenly. In the case of magnetic disk files, the prior art has also recognized the problem that magnetic guide rods may also offer a leakage path for magnetic flux to the neighborhood of the disks, thus affecting the read and write process. One solution to these problems is the use of non-magnetic stainless steel guide rods and a linear VCM employing such guide rods is described in U.S. Pat. No. 4,247,794.
The use of non-magnetic guide rails or bearings is also mentioned in an article by K. A. Mawla (IBM Technical Disclosure Bulletin, Vol. 28 No. 10, March 1986, p. 4479) entitled "Magnetic shunts to shield the bearings and rails of a voice coil motor actuator in a magnetic recording disk file", as a possible alternative to the use of such shunts.
An article entitled "Composite center core for reduction of stray magnetic fields in disk file actuators" by J. Cocke et al. (IBM Technical Disclosure Bulletin Vo. 28, No. 5, Oct. 1985, p. 1889) suggests a guide rod center core comprising a non-magnetic rod, such as stainless steel, soldered to a magnetic rod within the actuator stator.
A further problem with guide ways for linear actuators which is not discussed in the above referenced art is that of wear. Although the use of flat guide ways is known to reduce wear by spreading the load of the bearings, this requires precision alignment of the bearings with the flat guide surface which is difficult and costly to achieve. Thus in most recent disk files, round ways are preferred.
Wear of the carriage guide way surfaces, whether flat or round, can lead to tilting of the carriage in the direction of motion and consequent non-linear radial movement of the magnetic heads on the disk surfaces. Although such displacements can be accommodated to a limited extent by the head positioning servo system, the misregistration may exceed the limits that the servo can follow and, in any case, will limit the attainable track density.
The problem of tilt in linear actuator systems becomes more acute with smaller form factor disk files. This is because the wheelbase of the carriage is reduced and, by the principle of levers, the movement of the heads resulting from a small displacement of one of the bearings will be amplified.
Another aspect of wear which is particularly critical in disk files is that it is a source of particulate debris which can cause failure of the heads/disk interface where separations of 0.3 are typical.
As well as their better known uses as refractory or electrical insulating materials, ceramics such as zirconia have been proposed for a number of engineering applications involving wear under light loads or hot conditions. A number of such applications, as well as a general review of zirconia, are discussed in "An introduction to zirconia" by R. Stevens (Magnesium Elektron Ltd., 1986).
In a recent U.S. Pat. No. 4,709,284, zirconia is suggested as a material for the air bearing slider of a magnetic head because of its good sliding properties, abrasion resistance, high machinability and resistance to static electrical charging.